1. Field of the Invention
The present invention relates to an organic EL device, and more particularly, to an organic EL device and a driving apparatus thereof.
2. Discussion of the Related Art
Generally, an organic electroluminescent (EL) device is a device to emit light while a pair of electron and hole formed by injecting a charge into a light emitting layer of an organic material disposed between the electron injection electrode (cathode) and the hole injection electrode (anode).
Such an organic EL device is characterized by operation at a relatively low voltage and low power consumption.
Among the general organic EL devices, the organic EL devices for a unidirectional display can be classified into a bottom emission type and a top emission type.
A bottom emission type organic EL device is shown in FIG. 1. A method of fabricating the bottom emission type organic EL device shown in FIG. 1 will now be described.
An anode 2 is formed on a transparent substrate 1. The anode 2 is generally made of indium tin oxide (ITO).
Next, a hole injecting layer (HIL) 3 is formed on the anode 2. The HIL 3 is generally made of copper phthalocyanine (CuPc) and is coated to a thickness of 10-30 nm. The structure of CuPc is shown in FIG. 8 for your understanding.
Next, a hole transport layer (HTL) 4 is formed on the HIL 3.
The HTL is generally made of TPD (N′-diphenyl-N,N′-bis(3-methylphenyl)-(1-1′-biphenyl)4,4′-diamine) or NPD (4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl), and is deposited to a thickness of 30-60 nm. The structures of TPD and NPD are shown in FIG. 8 for your understanding.
Next, an organic emitting layer 5 made of an organic material is formed on the HTL 4.
The organic emitting layer 5 as formed may contain dopants if necessary.
In case of green light emission, Alq3 {tris(8-hydroxy-quinolate)aluminum} is deposited to about 30-60 nm so as to form the organic emitting layer 5, and coumarin 6 or Qd (Quinacridone) is added as the dopants.
In case of red light emission, ECM, DCJT, DCJTB or the like is used. The structure of Alq3 is also shown in FIG. 8.
Next, an electron transport layer (ETL) 6 and an electron injecting layer (EIL) 7 are continuously formed on the organic emitting layer 5, or the ETL 6 and the EIL 7 are formed together with an electron injection transport layer.
At this time, the EIL 7 is formed by coating LiF or Li2O to a thickness of about 5□, or by depositing alkali metal or alkaline-earth metal such as Li, CA, Mg, Sm, etc. to a thickness less than 200□ for making electron injection better.
Also, in case of the green light emission, since Alq3 used as the organic emitting layer 5 has a good electron transport capability, the EIL 7 and the ETL 6 may be not used.
Next, aluminum (Al) is coated on the EIL 7 to a thickness of about 1000 □ to form a cathode 8.
In the bottom emission type organic EL device formed by the above method, the material layer used as the cathode serves as a mirror reflection surface.
Accordingly, half of the light emitting from the emitting layer of the organic EL device emits toward the transparent electrode (anode) 2.
The remaining half of the light is reflected by the cathode and emits toward the transparent electrode (anode).
Next, a top emission type organic EL device is shown in FIG. 2. A method of fabricating the top emission type organic EL device shown in FIG. 2 will now be described.
Unlike in the bottom emission type, in the bottom emission type, an organic EL device is formed on an opaque substrate and a transparent electrode is lastly formed such that light emits in an opposite direction to the substrate.
An anode 12 is first formed on an opaque substrate 11.
Next, hole injecting layers (HILs) 13 and 14 are formed on the anode 12.
Next, an organic emitting layer 15 made of an organic material is formed on the HTL 14.
Next, an electron transport layer (ETL) 16 and an electron injecting layer (EIL) 17 are formed on the organic emitting layer 15.
Next, a transparent cathode 18 is formed on the EIL 17 such that light is irradiated upward.
FIG. 3 is a sectional view of a bidirectional organic EL according to the related art.
The organic EL device for bidirectional display employs the bottom emission type shown in FIG. 1 at the lower side from an emitting layer 25 and the top emission type shown in FIG. 2 at the upper side from the emitting layer 25 such that light emits in both directions of upper and lower sides.
However, the related art organic EL device for bidirectional display has the following drawbacks.
First, since light generated by a single emitting layer emits in upper and lower directions, the organic EL device for bidirectional display needs a power at least four times greater than that of the bottom emission type organic EL device so as to obtain the same brightness.
Secondly, since the organic EL device for bidirectional display is driven by a pair of electrodes, it is possible to display an identical picture in the upper and lower directions, but it is impossible to display different pictures on the upper screen and the lower screen.